Toidy is baby talk for (that is, a mispronunciation of) the word "dirty." The word is frequently associated with toilets, child defecation, and, by extension, the genitals. When an infant innocently plays with its feces or genitals, parents may implore the child to stop, saying "Dirty! That's dirty." The child mimics the parent's word, "dirty," but, lacking full and proper pronunciation, it sounds like "toidy."

As a little boy, I thought the word for penis was "toidy" because every time I touched my privates, my Dad would say "Stop it. That's toidy!"